I. Field of the Invention
The present invention relates to data communication. More particularly, the present invention relates to a novel and improved method and apparatus for the forward link power control in a communication system.
II. Description of the Related Art
The use of code division multiple access (CDMA) modulation techniques is one of several techniques for facilitating communications in which a large number of system users are present. Other multiple access communication system techniques, such as time division multiple access (TDMA) and frequency division multiple access (FDMA) are known in the art. However, the spread spectrum modulation techniques of CDMA have significant advantages over other modulation techniques for multiple access communication systems. The use of CDMA techniques in a multiple access communication system is disclosed in U.S. Pat. No. 4,901,307, entitled xe2x80x9cSPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS,xe2x80x9d assigned to the assignee of the present invention and is incorporated by reference herein. The use of CDMA techniques in a multiple access communication system is further disclosed in U.S. Pat. No. 5,103,459, entitled xe2x80x9cSYSTEM AND METHOD FOR GENERATING SIGNAL WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM,xe2x80x9d also assigned to the assignee of the present invention and is incorporated by reference herein. Furthermore, the CDMA system can be designed to conform to the xe2x80x9cTIA/EIA/IS-95-A Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular Systemxe2x80x9d, hereinafter referred to as the IS-95-A standard or TIA/EIA/IS-95A.
CDMA, by its inherent nature of being a wideband signal, offers a form of frequency diversity by spreading the signal energy over a wide bandwidth. Therefore, frequency selective fading affects only a small part of the CDMA signal bandwidth. Space or path diversity is obtained by providing multiple signal paths through simultaneous links to a mobile user or remote station through two or more base stations. Furthermore, path diversity may be obtained by exploiting the multipath environment through spread spectrum processing by allowing signals arriving with different propagation delays to be received and processed separately. Examples of path diversity are illustrated in U.S. Pat. No. 5,101,501 entitled xe2x80x9cMETHOD AND SYSTEM FOR PROVIDING A SOFT HANDOFF IN COMMUNICATIONS IN A CDMA CELLULAR TELEPHONE SYSTEM,xe2x80x9d and U.S. Pat. No. 5,109,390 entitled xe2x80x9cDIVERSITY RECEIVER IN A CDMA CELLULAR TELEPHONE SYSTEM,xe2x80x9d both assigned to the assignee of the present invention and incorporated by reference herein.
The reverse link refers to a transmission from a remote station to a base station. On the reverse link, each transmitting remote station acts as an interference to other remote stations in the network. Therefore, the reverse link capacity is limited by the total interference due to transmissions from other remote stations. The CDMA system increases the reverse link capacity by transmitting fewer bits, thereby using less power and reducing interference, when the user is not speaking.
To minimize interference and maximize the reverse link capacity, the transmit power of each remote station is controlled by three reverse link power control loops. The first power control loop adjusts the transmit power of the remote station by setting the transmit power inversely proportional to the received power on the forward link. In an IS-95-A system, the transmit power is given by pout=xe2x88x9273xe2x88x92pin where pin is the power received by the remote station given in dBm, pout is the transmit power of the remote station given in dBm, and xe2x88x9273 is a constant. This power control loop is often called the open loop.
The second power control loop adjusts the transmission power of the remote station such that the signal quality, as measured by the energy-per-bit-to-noise-plus-interference ratio Eb/Io, of the reverse link signal received at the base station is maintained at a predetermined level. This level is referred to as the Eb/Io set point. The base station measures the Eb/Io of the reverse link signal received at the base station and transmits a reverse link power control bit to the remote station on the forward traffic channel in response to the measured Eb/Io. The reverse power control bits are set 16 times per 20 msec frame, or at an 800 bps rate. The forward traffic channel carries the reverse link power control bits along with the data from the base station to the remote station. This second loop is often called the inner closed loop.
The CDMA communication system typically transmits packets of data as discrete data frames. Thus, the desired level of performance is typically measured by the frame-error-rate (FER). The third power control loop adjusts the Eb/Io set point such that the desired level of performance, as measured by the FER, is maintained. The required Eb/I0 to obtain a given FER depends upon the propagation conditions. This third loop is often called the outer closed loop. The power control mechanism for the reverse link is disclosed in detail in U.S. Pat. No. 5,056,109, entitled xe2x80x9cMETHOD AND APPARATUS FOR CONTROLLING TRANSMISSION POWER IN A CDMA CELLULAR MOBILE TELEPHONE SYSTEMxe2x80x9d, assigned to the assignee of the present invention and incorporated by reference herein.
The forward link refers to a transmission from a base station to a remote station. On the forward link, the transmission power of the base station is controlled for several reasons. A high transmission power from the base station can cause excessive interference with the signals received at other remote stations. Alternatively, if the transmission power of the base station is too low, the remote station can receive erroneous data transmissions. Terrestrial channel fading and other known factors can affect the quality of the forward link signal as received by the remote station. As a result, each base station attempts to adjust its transmission power to maintain the desired level of performance at the remote station.
Power control on the forward link is especially important for data transmissions. Data transmission is typically asymmetric with the amount of data transmitted on the forward link being greater than on the reverse link. With an effective power control mechanism on the forward link, wherein the transmission power is controlled to maintain the desired level of performance, the overall forward link capacity can be improved.
A method and apparatus for controlling the forward link transmission power is disclosed in U.S. Pat. No. 6,835,209, entitled xe2x80x9cMETHOD AND APPARATUS FOR PERFORMING FAST FORWARD POWER CONTROL IN A MOBILE COMMUNICATION art SYSTEMxe2x80x9d, hereinafter the ""209 patent, filed Mar. 31, 1995, now abandoned, assigned to the assignee of the present invention and incorporated by reference herein. In the method disclosed in the ""209 patent, the remote station transmits an error-indicator-bit (EIB) message to the base station when a transmitted frame of data is received in error. The EIB can be either a bit contained in the reverse traffic channel frame or a separate message sent on the reverse traffic channel. In response to the EIB message, the base station increases its transmission power to the remote station.
One of the disadvantages of this method is the long response time. The processing delay encompasses the time interval from the time the base station transmits the frame with inadequate power to the time the base station adjusts its transmission power in response to the error message from the remote station. This processing delay includes the time it takes for (1) the base station to transmit the data frame with inadequate power, (2) the remote station to receive the data frame, (3) the remote station to detect the frame error (e.g. a frame erasure), (4) the remote station to transmit the error message to the base station, and (5) the base station to receive the error message and appropriately adjust its transmission power. The forward traffic channel frame must be received, demodulated, and decoded before the EIB message is generated. Then the reverse traffic channel frame carrying the EIB message must be generated, encoded, transmitted, decoded, and processed before the bit can be used to adjust the transmit power of the forward traffic channel.
Typically, the desired level of performance is one percent FER. Therefore, on the average, the remote station transmits one error message indicative of a frame error every 100 frames. In accordance with the IS-95-A standard, each frame is 20 msec long. This type of EIB based power control works well to adjust the forward link transmit power to handle shadowing conditions, but due to its slow speed is ineffective in fading except in the slowest fading conditions.
A second method for controlling the forward link transmission power utilizes the Eb/Io of the received signal at the remote station. Since the FER is dependent on the Eb/Io of the received signal, a power control mechanism can be designed to maintain the Eb/Io at the desired level. This design encounters difficulty if data is transmitted on the forward link at variable rates. On the forward link, the transmission power is adjusted depending on the data rate of the data frame. At lower data rates, each data bit is transmitted over a longer time period by repeating the modulation symbol as described in TIA/EIA/IS95-A. The energy-per-bit Eb is the accumulation of the received power over one bit time period and is obtained by accumulating the energy in each modulation symbol. For an equivalent amount of Eb, each data bit can be transmitted at proportionally less transmission power at the lower data rates. Typically, the remote station does not know the transmission rate a priori and cannot compute the received energy-per-bit Eb until the entire data frame has been demodulated, decoded, and the data rate of the data frame determined. Thus, the delay of this method is about that described in the aforementioned U.S. Pat. No. 6,035,209 and the rate is one power control message per frame. This is in contrast with the reverse link approach in which there can be one power control message (bit) sixteen times per frame as in TIA/EIA/IS-95-A.
Other methods and apparatus for performing fast forward link power control are described in the aforementioned U.S. Pat. No. 6,035,209, U.S. patent application Ser. No. 08/559,386, entitled xe2x80x9cMETHOD AND APPARATUS FOR PERFORMING FAST FORWARD POWER CONTROL IN A MOBILE COMMUNICATION SYSTEMxe2x80x9d, filed Nov. 15, 1995, U.S. Pat. No. 5,903,554, entitled xe2x80x9cMETHOD AND APPARATUS FOR MEASURING LINK QUALITY IN A SPREAD SPECTRUM COMMUNICATION SYSTEMxe2x80x9d, filed Sep. 27, 1996, U.S. Pat. No. 5,893,035, entitled xe2x80x9cCENTRALIZED FORWARD LINK POWER CONTROLxe2x80x9d, filed Sep. 16, 1996, and U.S. Pat. No. 6,075,974, entitled xe2x80x9cMETHOD AND APPARATUS FOR ADJUSTING THRESHOLDS AND MEASUREMENTS OF RECEIVED SIGNALS BY ANTICIPATING POWER CONTROL COMMANDS YET TO BE EXECUTEDxe2x80x9d, filed Nov. 20, 1996, all are assigned to the assignee of the present invention and incorporated by reference herein.
The fundamental difference between the forward link and the reverse link is that the transmission rate does not need to be known on the reverse link. As described in the aforementioned U.S. Pat. No. 5,056,109, at lower rates, the remote station does not transmit continuously. When the remote station is transmitting, the remote station transmits at the same power level and the same waveform structure regardless of the transmission rate. The base station determines the value of a power control bit and sends this bit to the remote station 16 times per frame. Since the remote station knows the transmission rate, the remote station can ignore power control bits corresponding to times when it was not transmitting. This permits fast reverse link power control. However, the effective power control rate varies with the transmission rate. For TIA/EIA/IS-95-A, the rate is 800 bps for full rate frames and 100 bps for 1/8 rate frames.
An alternative reverse link architecture is described in the U.S. Pat. No. 5,930,230, entitled xe2x80x9cHIGH DATA RATE CDMA WIRELESS COMMUNICATION SYSTEMxe2x80x9d, hereinafter the ""230 patent, filed May, 28, 1996, assigned to the assignee of the present invention and incorporated by reference herein. In accordance with the ""230 patent application, an auxiliary pilot is introduced into the reverse link. The pilot level is independent of the transmission rate on the reverse link. This permits the base station to measure the pilot level and to send the reverse link power control bit to the remote station at a constant rate.
The present invention is a novel and improved method and apparatus for high rate forward link power control. The present invention improves the response time of the forward link power control loop and allows for dynamic adjustment of the transmission power on the forward link by measuring the quality of the reverse link power control bits which are transmitted on the forward traffic channel at multiple times within a frame. Measurements over short time intervals allow the base station to dynamically adjust the transmission power to minimize interference to other base stations and maximize the forward link capacity. The improved response time allows the power control loop to effectively compensate for slow fading. For fast fading, the block interleaver in the communication system is effective.
In accordance with the present invention, the remote station measures the reverse link power control bits which are transmitted at a rate of 800 bits per second on the forward traffic channel. The reverse link power control bits are punctured into the forward traffic channel data stream. The gain of the power control bits is adjusted along with the gain of the forward link data bits. However, unlike the data bits, the transmission level of the power control bit is not scaled according to the data rate. The measured signal quality of the power control bits is used to adjust the transmission power of the base stations.
It is an object of the present invention to improve the response time of the forward link power control by the use of the energy measurements of the reverse link power control bits. The reverse link power control bits are transmitted at a rate of 800 bps. Thus, the forward link power control mechanism of the present invention can perform a measurement of the quality of the received forward traffic channels periodically every 1.25 msec. The measurements can be transmitted to the base stations for use in adjustment of the forward link transmission power. The improved response time allows the base stations to effectively compensate for slow fades in the channel and improve the performance of the forward traffic channels.
It is another object of the present invention to increase the capacity of forward link by allowing for rapid adjustments in the transmission power of the base stations. The power control mechanism of the present invention allows the base stations to transmit at the minimal transmission power necessary to maintain the requisite level of performance. Since the total transmission power of the base stations is fixed, minimal transmission for a given task results in a saving of transmission power which can be used for other tasks.
It is yet another object of the present invention to provide for a reliable forward link power control mechanism. At the remote station, the reverse link power control bits from multiple sectors of a base station or multiple signal paths from the same sector are combined to yield an improved measurement of the forward link signal quality. The reverse link power control bits which are deemed unreliable may be omitted from use in the power control loop. At the base stations, the forward link power control bits are received by all base stations in communication with the remote station. The gains of the forward traffic channels of the base stations are corrected periodically so that erroneous reception of the forward link power control bits by the base stations do not accumulate.
It is yet another object of this invention to provide a mechanism to adjust the forward link power to the desired frame error rate, similar to that done by the outer loop for the reverse link.
It is yet another object of this invention to provide a mechanism to communicate the power control bits between base stations. The power control bits which control the forward link transmit power may or may not have been correctly received at different base stations. The present invention provides base stations which receive erroneous power control bits with the information necessary to update their forward link transmit power.